That is an Improvement" meaning you add the roof as an asset, not as a repair or expense. (unless it was a very small patch)
Assets are then depreciated, just like the building.
This year, as part of the Tax Cuts and Jobs Act, the Taxpayer (you) have the option of using Bonus Depreciation and/or the 179 deduction. This means that you can "Depreciate" or Deduct the full cost on your 2018 return (if eligible- must be purchased and put into use in 2018). or an adjusted amount, whichever would benefit you more.
You will be given options when you enter the asset (roof) into the TurboTax program.
